In a typical ion implanter, a relatively small cross-section beam of dopant ions is scanned relative to a wafer or other substrate. This can be done in one of three ways: scanning of the beam in two directions relative to a stationary wafer, scanning of the wafer in two directions relative to a stationary beam, or a hybrid technique in which the beam is scanned in one direction while the wafer is mechanically scanned in a second direction. Typically, the two directions are orthogonal.
Batch implanters typically have a rotating wheel carrying a number of wafers around its periphery, and are designed for the simultaneous implantation of a batch of wafers in a process chamber of an implanter. The wheel is rotated so that each wafer passes through the ion beam in turn repeatedly as the axis of rotation is translated such that the ion beam traces across the entire wafer in a series of scan lines. An example of such a scanning wheel type batch implanter is disclosed in U.S. Pat. No. 5,389,793.
Some single-wafer implanters scan an ion beam in two directions relative to a stationary wafer, see for example U.S. Pat. No. 4,736,107. Other single-wafer implanters adopt hybrid techniques where the ion beam is scanned in one direction and the wafer mechanically scanned in an orthogonal direction, see for example U.S. Pat. No. 5,898,179, U.S. Pat. No. 5,003,183, U.S. Pat. No. 5,229,615 and U.S. Pat. No. 5,406,088.
Other single-wafer implanters scan the wafer in two directions relative to a fixed ion beam. Scanning arms that hold a wafer and effect linear translation of the wafer in two axes are well known. Typically, a cantilevered arm is mounted to extend horizontally from a wall of the process chamber. The arm is driven by linear actuators in both horizontal (x-axis) and vertical (y-axis) directions, the ion beam defining the z-axis. A raster scan is implemented relative to these directions as follows. The x-axis direction along the scanning arm is the fast-scan direction, i.e. the scanning arm is scanned to and fro to form successive scan lines on the wafer. The y-axis direction transverse to the scanning arm is the slow-scan direction, i.e. the scanning arm is moved in this direction after each scan line so as to cause the scan lines to be spaced uniformly.
An alternative scanning arm is described in WO2004/001789 and is shown herein in FIG. 1. A wafer 10 is scanned using rotational movements of an arm 1 provided with an elbow 30. The arm 1 is cantilevered from a shoulder 20, and both shoulder 20 and elbow 30 may be rotated about parallel axes 21 and 31 that extend in the same direction as the ion beam 50. Rotation is effected by a pair of motors, with one motor 22 being provided at the shoulder 20 and the other motor 32 being provided at the elbow 30.
The motors 22 and 32 are driven in opposite senses, as indicated by arrows 23 and 33, so as to move the wafer 10 horizontally through the ion beam 50 to form successive scan lines. In addition, motors 22 and 32 may be used to move the wafer 10 vertically between scan lines. A further motor 12 is provided at the distal end of scanning arm 1 so as to rotate the wafer 10 on its holder 14 about is central axis 11, i.e. about direction 13. This allows the orientation of the wafer 10 relative to the ion beam 50 to be kept constant as the shoulder 20 and elbow 30 are rotated to effect scanning of the wafer 10.
FIG. 1 shows the wafer 10 held perpendicularly to the ion beam 50. Implants may also be performed at other angles of incidence by virtue of a further motor 42 that rotates a boss 40 about axis 41. The shoulder 20 is mounted to the boss 40, such that rotation of the boss 40 in direction 43 causes the wafer 10 to tilt relative to the ion beam 50.
The scanning arm of FIG. 1 suffers from some disadvantages. Motor 32 has substantial mass that is located at the elbow 30 some distance away from the shoulder 20. This places a high torque requirement on motor 22 to move this cantilevered mass as it drives the scanning arm 1. In addition, the cantilevered mass of the motor 32 tends to produce vibrations in the scanning arm 1. These vibrations are transmitted to the wafer 10 leading to a lack of uniformity in the dose received across the wafer 10.